1. Field of the Invention
The present invention is related to paging in distributed communications systems, and, more particularly, to techniques and systems for paging in portable personal communications systems characterized by a limited number of communication channels between mobile and base units, such as personal handyphone (PHS) and micro-cellular systems.
2. Description of Related Art
Portable personal communication systems have enjoyed a dramatic increase in popularity in recent years, and the trend is sure to continue. With their ability to place conventional telephone calls from virtually anywhere in a geographically diverse environment, their high quality of service and their compact size, such systems provide users with convenience, flexibility and ease of use unparalleled by other communication systems.
Portable personal telephone networks such as the one shown in FIG. 1 typically include three main parts: a number of portable telephones 10 (hereinafter "Personal Communication System" or "PCS units"), several cell stations 12 each disposed within a respective coverage zone 14, and a PCS network controller 16 (hereinafter "PNC"). The PCS units 10 communicate exclusively with the cell stations 12, and the cell stations 12 communicate with the PCS units 10 and the PNC 16.
The Federal Communications Commission (hereinafter "FCC") has allocated 666 RF channels for communications between PCS units 10 and cell stations 12. Of these channels, 312 in each band are used for voice and data transmission (hereinafter, these channels will simply be called "voice channels") and the remaining 21 are reserved for use as setup channels, the significance of which will be described below.
In practice, a multiplexing technique is used so that each RF channel can be used to service three PCS units 10 simultaneously; however, in the following discussion, for simplicity it will be assumed that each RF channel is dedicated to a single PCS unit 10.
One of the biggest advantages of PHS systems is their relatively inexpensive parts and their consequent low cost. Thus, it is advantageous for the cell stations to handle as few channels as possible, since the cost and complexity of a cell station increases with the number of channels it services.
The PNC 16 is the system's link to the conventional telephone network. When a PCS unit 10 originates or receives a call, a radio link is established between the PCS unit 10 and the cell station 12, and a data link (typically an ISDN connection) is established between the cell station 12 and the PNC 16. In this way, the PNC 16 establishes a path between the PCS 10 and the conventional wireline telephone network.
The process of establishing a call with a PCS unit 10 will now be described in detail. When the PCS unit 10 is turned on, it scans a group of setup channels whose frequencies have previously been stored in its memory and selects the RF channel corresponding to the best signal (e.g., the strongest signal) as shown in Step 100 of FIG. 2. It sets this RF channel as its operative setup channel in Step 102, receives a local identification code (hereinafter "LID") periodically sent by the cell station 12 transmitting on the operative setup channel in Step 104, and compares that LID to the LID of its last known position in Step 106.
If the two LIDs are different, the PCS unit 10 determines that it has moved to a new area since its last position registration and proceeds to register its new position with the current cell station 12 via the operative setup channel in Step 108. If the two LIDs are identical, the system loops through Steps 110 and 112 to wait for an incoming call from the registered cell station 10 (also known as paging) in Step 110 or for the user to place an outgoing call (also known as access) in Step 112. While waiting to execute the paging and access procedures, the PCS unit 10 checks to see if the signal from the operative setup channel has fallen below a predetermined threshold in Step 114 and if so, the PSC unit 10 executes the scanning procedure of Steps 100-104 again to establish a new operative setup channel.
When a user attempts to initiate access and place an outgoing call from the PCS unit 10, the PCS unit 10 sends a connection request signal, including the telephone number to be called, asking for a connection on a free voice channel to the operative cell station 12 as shown in Step 116. The cell station 12 sends a response signal notifying the PCS unit 10 of the assigned voice channel which is received by the PCS unit 10 in Step 118, the cell station establishes an ISDN link with the PNC 16 to connect to the wireline telephone network, and the call begins. The PCS unit 10 then places the outgoing call with the cell station 12 in Step 120 and loops through Step 122 waiting for the user to terminate the call, and at that point sends a disconnection request signal to the cell station 12 to free the allocated voice channel in Step 124 and returns to the paging-access loop of Steps 110 and 112.
In the complementary paging process, when the PNC 16 receives a call destined for a PCS unit 10 from the wireline telephone network, it sends a paging message containing the called number to all of its cell stations 12, and each cell station 12 broadcasts the called number on all of its setup channels. Since the PCS unit 10 is monitoring the operative setup channel, it recognizes its number in Step 110 and sends a connection request signal to the cell station 12 in Step 126. Then, the PCS unit 10 receives a response signal notifying the PCS unit 10 of the assigned voice channel in Step 128, receives the incoming call on the assigned voice channel in Step 130, loops through Step 132 waiting for the user to terminate the call, and sends a disconnection request signal in Step 134 as in the case of the access process described above.
One other connection procedure, "handing off", is essential to PHS operation. As noted above, if while waiting for a paging or access procedure a PCS unit 10 determines that the signal strength of its operative setup channel has fallen to an unacceptably low level, it will scan the setup channels to find a better one. However, while a call is in progress, the cell station 12 may determine that the setup channel signal from the PCS unit 10 has fallen to an unacceptably low level. This is most often the result of movement of the PCS unit 10 away from the operative cell station 12, i.e., "roaming". In this case, the cell station 12 will notify the PNC 16 of the deterioration of the signal from the PCS unit 10.
The PNC 16 then sends a monitor request signal to other cell stations 12 adjacent to the operative cell station 12 instructing them to assess the strength of the setup channel signal from the PCS unit 10. The other cell stations 12 report their results, and the PNC 16 sends a command to the PCS unit 10 via the operative cell station 12 instructing it to make the other cell station 12 having the highest received signal strength its new operative cell station and to switch its voice channel accordingly.
The above system works well; however, it has practical disadvantages. For example, as noted above, it is advantageous for cell stations to use as few RF channels as possible. However, reducing the number of cell station channels increases the likelihood of call blocking, i.e., a situation where a PCS unit 10 attempting access to a cell station 12 or where a cell station 12 attempting to page a PCS unit 10 finds all voice channels occupied. If an access operation from a PCS unit 10 or a paging operation from a cell station 12 is blocked and the PCS unit 10 is not within the coverage zone of another cell station 12 which has a free channel, the call will be lost. Of course, this reduces the grade of service of the system, and most PHS systems attempt to keep the proportion of blocked calls during their busy hours under 1%.
FIGS. 3 and 4 show examples of call blocking in prior art systems. A paging group coverage area 18 is formed by respective individual coverage areas 20a, 20b and 20c of cell stations 12a, 12b and 12c in FIG. 3. Calls are established between PCS units 10a-10c and cell station 12a; thus, cell station 12a is fully loaded (in this Figure, light arrows from a PCS unit to a cell station indicate a PCS unit registration, while heavy arrows from a cell station to a PCS unit indicate a connected call). Further, a call is established between PCS unit 10d and cell station 12b, and PCS unit 10e makes a request to place an outgoing call with cell station 12a. FIG. 4 shows a similar situation in which a call between PCS unit 10e and cell station 12b has previously been established; however, PCS unit 10e has roamed from the coverage area 20b to the coverage area 20a, and it requests that its call be handed off to cell station 12a.
In either situation, the call from PCS unit 10e will be blocked because cell station 12a is fully loaded. One way to avoid this problem while keeping the number of channels on each cell station 12 low is to add additional cell stations; however, this is expensive and there is a practical limit on how many cell stations can be placed in close proximity to one another while maintaining adequate frequency reuse conditions.